1. Field of the Invention
The invention relates to microwave transmission lines including active electronic components. More specifically, it relates to an apparatus for coupling a waveguide structure to a printed circuit transmission line carrying high frequency electronic components which are supplied by a direct current biasing voltage.
2. Description of the Prior Art
The most common mode of transmission at frequencies of 18 GHz and above is a waveguide system. Waveguide systems often employ active devices wherein the term "active device" herein is understood as an electronic device requiring a direct current bias. Such devices usually are implemented in the form of high frequency electronic devices in solid-state technology. It is, therefore, necessary to provide for transistion stages from a waveguide structure to a printed circuit transmission line, such as a slot line or a microstrip line including shielded microstrip lines with suspended substrates. Solid state devices are mounted onto the same substrate and are electrically connected to the transmission line. In a variety of applications two subsequent transition stages are necessary if the electronic device has to be inserted in between the run of the waveguide system.
Such transition stages for coupling a waveguide structure to a printed circuit transmission line are widely used and well known in the art, as may be seen for example, from an article "Millimeter-Wave IC Components Using Fine Grained Alumina Substrate" by H. Yatsuka et al, published in 1980 IEEE MTT's International Microwave Symposium Digest, pages 276-278. This article describes several passive IC components for use with waveguide systems. Passive components which do not require a direct current bias for operating are to be integrated relatively easily into a waveguide system, as long as there are provided matching networks for balancing impedances.
Active electronic devices, however, normally require direct current (D.C.) biasing voltage. In microwave applications the feeding circuitry of the direct current voltage has to be carefully designed, otherwise undesired interference with the radio frequency (RF) network will occur. Conventionally, for applications of a lower frequency range, a radio frequency choke and a low pass filter are used for connecting a direct current voltage source to an active device. For high frequency applications in the millimeter wave range, according to the different technology, a high impedance line connected to the radio frequency network and a printed circuit low pass filter for the direct current by-pass may be provided.
In addition, for blocking the DC voltage component, either a series cut capacitor, or a coupler is inserted into the path of the useful signal. Such an implementation is described and shown in the article "A K-Band 1 Watt GaAs FET Amplifier" by Sane et al, published in 1980 IEEE, MTT's International Symposium Digest, pages 180-182. It is evident from the description with reference to FIGS. 2 and 3 of this article that feeding of the DC bias voltages has considerable impact on the implementation of such an active device, since adding the DC biasing network and a blocking capacitor to the circuit usuallly causes mismatch problems requiring a complicated microstrip network. This design results in an increase of loss. It may be mentioned that the known apparatus overcomes the coupling problems by utilizing coax connectors. Since a transition to coax lines does not imply an electrically short-circuited contact, DC blocking is achieved without further efforts, but it has to be established with a two-stage transition, that is a first transition from the printed circuit transmission line to a coax line and a second transition from the coax line to the waveguide. Obviously, for transmissions along coax cable the DC blocking problem is of minor importance.
For these reasons, efforts have been made to overcome these restrictions with respect to waveguide systems. One approach is knwon from an article "20 GHz Band GaAs FET-Waveguide-Type Amplifier" by Hideki Tohyama, published in 1977 IEEE MTT's International Microwave Symposium Digest June 21-23, 1977, San Diego, Calif. describing an arrangement wherein the active device is integrated into the waveguide structure. An integration scheme, as shown in FIG. 3 of the last-mentioned article, has the disadvantage that any design is specifically limited to a particular application. The lumped element structure mounted directly into a waveguide therefore, is of limited interest with respect to coupling various and more complex, active devices to a waveguide system, in contrast to printed circuit transmission lines which do not show this drawback. Furthermore, it is assumed that the known structure which is proven at 20 GHz may also have limitations for transmitting signals of high frequencies in terms of smaller gains and less feasibility. Mounting passive and active electronic devices onto a printed circuit transmission line and providing for a low loss transition to the waveguide, therefore, still seems to be the most feasible approach.